Coaxial impact collision ion scattering spectrometers (CAICISS) are widely used in researching semiconductors and new materials to analyze chemical composition or crystalline structure of a sample especially in its surface.
In a CAICISS, a beam of ions having known mass and known speed (that is, known kinetic energy) is irradiated from an ion source onto a sample. The irradiated ions elastically collide with the atoms of the sample mostly in its surface and are scattered, when the scattered ions lose a part of their energy depending on the mass of the collided atom in the sample surface. Among the scattered ions, those scattered backward are detected by an ion detector placed in the same direction as the ion source in relation to the sample surface (that is, the ion source and the ion detector are placed coaxial). By measuring the energy of the back-scattered ions, the surface of the sample can be analyzed. In a time-of-flight type CAICISS, the energy of the back-scattered ions is measured by their speed, and the speed is measured by the time of flight until the ions are detected by the ion detector.
Here it is apparent that the correct analysis of the sample needs the exact knowledge of the mass and energy of the irradiated ions. The ions irradiated onto the sample are generated in the ion source by ionizing a source gas. One of the problems of prior art CAICISSs is neutral atoms included in the ions irradiated onto the sample. While not all the source gas is ionized in the ion source, the non-ionized atoms come out of the ion source by the thermal motion. Prior art CAICISSs have a straight path from the ion source to the sample surface and the ion beam continuously ejected from the ion source is chopped by chopping electrodes, whereby the ions are intermittently irradiated onto the sample and the time of flight is measured distinctly. Since the non-ionized atoms are electrically neutral, they are not chopped by the chopping electrodes but are continuously irradiated onto the sample.
Another problem is impurity ions inevitably included in the ions generated in the ion source. When the source gas is ionized, residual gas in the ion source, impurity gas in the source gas, or vapor gas from a heater filament are also ionized. In prior art CAICISSs, all the ions including such impurity ions are ejected from the ion source and irradiated onto the sample.
In these cases, the energies of the impurity ions or neutral atoms irradiated onto the sample are unknown, whereby the impurity ions and neutral atoms scattered by the sample surface and detected by the detector make noise and build higher background in detection signals of the ion detector.